The present invention relates to a circuit and method for sampling an output signal to perform automatic gain control (AGC) in a lowspeed data receiver.
Infrared wireless data communication is a useful method for short range (in the approximate range of 0-10 meters) wireless transfer of data between electronic equipment; such as, cellular phones, computers, computer peripherals (printers, modems, keyboards, cursor control devices, etc.), electronic keys, electronic ID devices, and network equipment. Infrared wireless communication devices typically have the advantages of smaller size, lower cost, fewer regulatory requirements, and a well defined transmission coverage area as compared to radio frequency wireless technology (i.e. the zone of transmission is bounded by physical walls and therefore more useful in an office environment). In addition, infrared wireless communication has further advantages with regard to reliability, electromagnetic compatibility, multiplexing capability, easier mechanical design, and convenience to the user as compared to cable based communication technology. As a result, infrared data communication devices are useful for replacing 0-10 meter long data transfer cables between electronic devices, provided that their size and costs can be reduced to that of comparable cable technology.
Infrared data communications devices typically consist of transmitter and receiver components. The infrared data transmitter section consists of one or more infrared light emitting diodes (LEDs), an infrared lens, and an LED current driver. A conventional infrared data receiver typically consists of an infrared photodiode and a high gain receiver amplifier with various signal processing functions, such as automatic gain control (AGC), background current cancelling, filtering, and demodulation. For one-directional data transfer, only a transmitter at the originating end and a receiver at the answering end is required. For bi-directional communication, a receiver and transmitter at each end is required. A combined transmitter and receiver is called a transceiver.
In typical high volume applications, it is now standard practice to fabricate the receiver circuitry and transmitter driver in a single integrated circuit (IC) to produce a transceiver IC. In turn, a transceiver IC, infrared photodiode and LED along with lenses for the photodiode and LED are assembled together in a plastic molded package designed to be small in size and allow placement in the incorporating electronic device so as to have a wide angle of view (typically through an infrared window on its case). The transceiver IC is designed to digitally interface to some type of serial data communications device such as an Infrared Communication Controller (ICC), UART, USART, or a microprocessor performing the same function.
A representative example of a conventional infrared data transmitter and receiver pair is shown in FIG. 1. Infrared transmitter 10 includes LED 16 which generates a modulated infrared pulse in response to transistor 14 being driven by the data signal input at D.sub.IN. The modulated infrared signal is optically coupled to an infrared detector, such as photodiode 24 normally operated in current mode (versus voltage mode) producing an output current which is a linear analog of the optical infrared signal falling on it. The infrared pulses generated by LED 16 strike photodiode 24 causing it to conduct current responsive to the data signal input at D.sub.IN thereby generating a data signal received at D.sub.IR.
In receiver 20, the signal received at D.sub.IR is transformed into a voltage signal V.sub.IR and amplified by amplifier 26. The signal output from amplifier 26 then feeds into comparator 42 which demodulates the received signal by comparing it to a detection threshold voltage V.sub.DET in order to produce a digital output data signal at D.sub.OUT.
The received signal waveform will have edges with slope and will often include a superimposed noise signal. As a result, V.sub.DET is ideally placed at the center of the received signal waveform so that the output data signal has a consistent waveform width despite the slope of the received signal edges. Also, placing V.sub.DET at the center of the received signal improves the noise immunity of receiver 20 because the voltage difference between V.sub.DET and both the high and low levels of the received signal is maximized such that noise peaks are less likely to result in spurious transitions in V.sub.OUT.
The received signal, however, can vary in amplitude by several orders of magnitude due primarily to variations in the distance between transmitter 10 and receiver 20. The strength of the received signal decreases proportional to the square of the distance. Depending on the range and intensity of the infrared transmitter, the photodiode outputs signal current in the range of 5na to 5ma plus DC and AC currents arising from ambient infrared sources such as sunlight and both incandescent and fluorescent lighting. As a consequence, the center of the received signal waveform will vary, whereas V.sub.DET must generally be maintained at a constant level. To address this problem, receivers typically include an automatic gain control (AGC) mechanism to adjust the gain responsive to the received signal amplitude. The received signal is fed to AGC peak detector 36 which amplifies the signal and drives current through diode 32 into capacitor 28 when the signal exceeds the AGC threshold voltage V.sub.AGC in order to generate a gain control signal. The gain control signal increases in response to increasing signal strength and correspondingly reduces the gain of amplifier 26 so that the amplitude of the received signal at the output of amplifier 26 remains relatively constant despite variations in received signal strength.
At a minimum, infrared receiver 20 amplifies the photodetector signal current and then level detects or demodulates the signal when it rises above the detect threshold V.sub.DET thereby producing a digital output pulse at D.sub.OUT. For improved performance, the receiver may also perform the added functions of blocking or correcting DC and low frequency AC ambient (1-300 ua) signals and Automatic Gain Control (AGC) which improves both noise immunity and minimizes output pulse width variation with signal strength.
Data can be modulated on the infrared transmitted signal by a number of well known methods. One popular method is defined by the Infrared Data Association (IrDA). IrDA Physical Layer Link Specification 1.1e specifies two main physical layer infrared modulation methods. One method is a low-speed (2 Kbp/s to 1.15 Mbp/s) on-off infrared carrier asynchronous modulation where the presence of a pulse indicates a 0 bit and the absence of a pulse indicates a 1 bit. The second method is a high speed (4 Mb/s) synchronous Four Pulse Position Modulation (4 PPM) method in which the time position of a 125 ns infrared pulse in a 500 ns frame encodes two bits of information.
Because there is ramping on the received waveform V.sub.IR, which can cause widening or narrowing of the signal pulse unless the detect threshold V.sub.DET is in the center of the waveform, AGC improves the fidelity of the output pulse by maintaining V.sub.DET at the center of the waveform. The high speed 4 PPM protocol is highly sensitive to pulse width distortion which requires additional complex circuitry to correct. However, the low speed IrDA protocol can function with relatively poor pulse width fidelity and can tolerate pulse width variations of more than three to one without impairment of the demodulation function in the receiver. Thus, the low speed IrDA protocol can be implemented with simpler circuitry.
Low speed transceivers (2.4 Kbits/sec to 115 Kbits/sec) represent a potentially high volume market having high cost sensitivity. Thus, it is particularly desirable to produce receivers at the lowest cost possible. Increased cost is generally associated with increased circuit complexity because complex circuits typically use more components which require more integrated circuit area, have lower yields in fabrication due to lower probability that all components will be functioning correctly, and are usually more time consuming to test. Because low speed protocols can tolerate lower pulse fidelity, low speed transceivers can be designed with relatively simple circuit designs that still yield adequate performance.
One effective circuit design method for designing a relatively simple receiver is described in the commonly-assigned patent application referenced above entitled "APPARATUS AND METHOD FOR SUPPRESSION OF FEEDBACK IN A COMMUNICATIONS RECEIVER", wherein the receiver is designed such that the feedback signal from the output terminal of the receiver to the input terminal is in-phase with the signal received at the input terminal. In this manner, it is possible for receivers which demodulate on-off modulation, as specified by IrDA, to receive signals significantly below the feedback transient amplitude provided that the receiver transient response has little overshoot and either no AGC or high signal threshold AGC is used. Under these circumstances the feedback acts as dynamic hysteresis, producing a pulse without spurious transitions.
One possible limitation of the in-phase feedback method is that in order to prevent AGC desensitization, the AGC threshold needs to be set well above the peak feedback value by a safe tolerance. Although setting the AGC threshold at a high level results in significant variation in detected pulse width, this variation may be acceptable in lowspeed IrDA compatible applications since bit information is encoded by the presence or absence of a pulse and not by its width, so long as the pulse does not widen so much as to interfere with the adjacent pulse window.
Another undesirable limitation of the in-phase feedback method is feedback transient overshoot or ringing, which, if it exceeds the detect level, will cause undesirable extraneous output pulse transitions. Although the use of well known filter design techniques can theoretically limit transient overshoot to any arbitrarily small value, in practice, reducing it to a value below 1/5 or 1/10 the peak level is difficult due to variable phase shift effects internal and external to the infrared receiver. Some of these variable phase shift effects are due to normal variances in such factors as transmit pulse shape, photodiode time constant, photodiode capacitance, receiver supply voltage, filter component values, etc.
Despite these limitations, the in-phase feedback method can beneficially decrease the disruptive effects of feedback by 10 db-20 db for infrared receivers used with edge-triggered serial data communication controllers which do not need an accurate data pulse width or with receiver systems which do not require the benefits of a low threshold AGC.
In-phase feedback control still requires some shielding between the input and output terminals of the receiver since, without shielding, the feedback signal will still be on the order of 10 db-20 db above the minimum received signal. Therefore, some shielding will still be required if feedback mitigation is not accomplished by the receive circuit. However, shields represent a major cost factor. To operate without a shield, the receiver must tolerate feedback levels which are as high as 30 db-40 db above the minimum received signal.
Therefore, the need remains for a relatively simple receiver design which mitigates feedback and tolerates a feedback signal that is 30 db-40 db above the minimum received signal so that the receiver can be constructed with a small size and without a shield.